President Barack Obama's lack of leadership on the world stage has left America's allies without support and enabled instability to escalate, Sen. Bob Corker said.

In a commentary for The Washington Post, the Tennessee Republican and ranking member on the Senate Foreign Relations Committee, said the president's rhetoric doesn't match his actions, and America's allies have suffered as a result.

"Those around the world who are looking to the United States for support against intimidation, oppression, or outright massacres have learned a tough lesson in the past few years: This U.S. president, despite his bold pronouncements and moral posturing, cannot be counted on," he wrote.

In Syria, Corker said, the president made bold statements about ousting President Bashar al-Assad, but in the end did not help opposition groups defend themselves against the regime, or take action after Assad crossed the "red line" in using chemical weapons against his own people.

"The president asked Congress for the authorization to use military force against Syria. I worked with my colleagues to move that authorization out of the Foreign Relations Committee to the floor of the Senate. But when faced with the difficult challenge of persuading the rest of Congress to support his policy, Obama reversed course and said he no longer wanted the authority," Corker wrote.

As a direct result of the president's actions, Corker said, extremists have been able to thrive in Syria and Iraq, "threatening to completely undo the progress of years of U.S. sacrifice." The Islamic State group has seized large parts of Iraq and Syria.

Meanwhile, Corker said the president's "empty promises" and "unreliability" have been most evident in Eastern Europe.

"Our tepid response to Russian aggression in Ukraine for nearly five months emboldened Putin, directly undermining U.S. interests and making Europe, and thus the United States, less secure," he wrote. Russia annexed Crimea from Ukraine earlier this year. Russian President Vladimir Putin is now supporting pro-Russian separatists in Ukraine.

Corker wrote, "I support the president's desire to avoid 'stupid' wars, and to 'steadily advance the interests of the American people and our partnership with folks around the world,' but, time and again, this president proves that he is uncomfortable being commander-in-chief and implements policies unsteadily and at odds with his stated goals, further undermining our credibility with these very partners.

"More often than not, the president doesn't hit singles and doubles; he just balks."